Many electric power plants burn fossil fuels such as coal or natural gas in order to generate electricity. In the combustion process, various undesirable products are created including nitrous oxides (NOx), sulfur oxides (SOx), mercury (Hg), and particulate material (coal ash). These pollutants must be removed from the combustion gas stream to levels below those prescribed by the Environmental Protection Agency (EPA) before the “cleansed” gas is emitted from the plant stack. Various types of pollution control equipment are used to remove these chemical compounds and particulate. One such system is the Selective Catalytic Reactor (SCR), which removes the NOx from the gas stream through a catalytic reaction Ammonia (NH3) is injected into the NOx-laden gas stream and a reducing reaction occurs through the catalyst, converting the NOx to nitrogen (N2) and water vapor (H2O).
The design of an SCR system to reduce NOx emissions from these plants involves a number of engineering disciplines including chemical, mechanical, fluid dynamic, and structural engineering. With respect to the fluid dynamic design, there are a number of flow-related parameters that need to be addressed in order to optimize the SCR performance. It is vital to achieve a uniform distribution of velocity, temperature, NOx, and NH3 through the catalyst material while also minimizing the system pressure losses. Unfortunately, these goals often work counter to each other. For example, attaining uniform, streamlined velocity patterns and low pressure drop requires an aerodynamic design of the ductwork that conveys the gases from the combustion zone (the boiler) to the SCR. Aerodynamic elements, such as turning vanes, flow straighteners, and other devices are used to provide a smooth, controlled flow stream. This runs counter, however, to the requirement for uniform temperature, NOx, and NH3. These parameters benefit from turbulent, chaotic mixing of the gas species, so static mixing devices are often inserted into the gas stream to homogenize the flow.
Thus, an optimally-designed system requires a careful balance of these competing flow-related goals. In particular, the industry has found it challenging to mix the gaseous species of NOx and NH3 sufficiently without creating an adverse effect on the gas velocity uniformity and/or pressure drop. Static mixers are frequently used to induce the turbulence and the mixing action. Some mixers do this by creating a swirl or a rotational vortex. Other mixers divide the flow and angle it in different directions, resulting in shear layers where turbulent mixing occurs. All of these mixing concepts tend to cause the velocity patterns to become misaligned with the primary flow direction, generating angular or swirling flow vectors. This can have a negative impact on the other primary goal of achieving a smooth, uniform velocity distribution with no angular vector as the flow enters the catalyst. Thus, these types of mixers can require long distances for the flow to smooth out, or they require additional flow control devices (adding to cost and pressure drop) to re-align and distribute the flow. Some of these mixers are also overly-sensitive such that subtle changes to incoming flow conditions (i.e., NH3 injection locations or incoming NOx profile from the boiler) result in significantly different mixing behavior.
Though there are a number of different static mixers that have been developed for SCRs, they tend to fit into one of two categories based on the fluid-dynamic behavior that they induce, i.e., shear mixers and vortex mixers. Many shear mixers redirect flue gas in a manner that alters the flow significantly from its original direction thereby promoting significant angular velocity components that are not aligned with the duct direction. These flow angles can persist and may result in a non-uniform velocity distribution downstream of the static mixer at the catalyst. Unless the ducts have long lengths for the flow to redistribute, additional flow devices will be needed to control the flow and obtain the required velocity profile at the catalyst.
Also many, but not all, shear mixers generate shear in only one direction (i.e., length-wise of the duct cross section) as a primary focus. Often, a second mixer is required to promote shear in the other duct direction (i.e., width-wise). The need for multiple mixers to insure adequate mixing can require a relatively long length to ensure a proper level of mixing is achieved.
In addition to shear mixers, there are “vortex mixers”. Vortex mixers induce rotational eddies and vortices. These are often large plates or other bluff bodies located in the gas stream to block the flow and divert it. A large wake is created by these plates and thus a low pressure region exists on the downstream side. The NH3 is generally injected into this wake, downstream of the mixer, and the eddies created by the vortices induce NH3 mixing. This can provide quite reasonable NH3 and NOx mixing for SCR systems, but because of their nature the vortex mixers do not allow for much adjustability, or tuning of the NH3 if it is needed. Vortex mixers can also be sensitive to incoming flow conditions but have no mechanism which would allow for easy system adjustments.
For static mixers to perform well, meeting the ammonia and NOx mixing objectives, temperature mixing objectives, and pressure drop objectives often associated with SCR systems while avoiding negative influences on velocity distribution or angularity is highly desirable. In SCR systems such objectives have to be accomplished under what are often rather adverse conditions including the presence of erosive particulate in the flow stream (especially for coal-fired power plants), elevated temperatures, and varied incoming conditions (such as gas flow rate or NOx profile from the boiler). To be practical, in addition to meeting the functional objectives, static mixers should also be affordable and reasonably easy to install and maintain.
Thus it should be appreciated, that there is a need for new mixing devices which balance the competing flow-related goals of mixing one or more reagents with a gas flow, e.g., flue gas flow, without significantly impeding the flow while still achieving a desirable level of mixing.